It is proposed that factor VIII ligands can modulate the expression or development of inhibitory antibodies in hemophiliacs. There is evidence that non-inhibitory antibodies are produced which bind to factor VIII and we hypothesize that some of these may have the property of blocking the binding of inhibitors. The kinetic properties of the type II inhibitor response, in which inhibitory antibody can co-exist with active factor VIII, can be modeled by a mixture of inhibitory and blocking antibodies. Equilibrium mixtures of these antibodies and factor VIII would contain factor VIII activity levels dependent upon the concentrations of each of the antibody species and their respective dissociation constants. We will analyze the anti-factor VIII antibodies of the type II response and antibodies found in patients subsequent to factor VIII-induced tolerance. Additionally, murine monoclonal antibodies will be selected and analyzed for their ability to block inhibitors. A single chain variable fragment of a murine factor VIII blocking antibody will be tested as a model therapeutic agent. A second ligand which is known to modulate inhibitory expression and which may regulate immune response is von Willebrand factor. Changes in the binding affinity of factor VIII for von Willebrand factor both would alter the availability of free factor VIII to inhibitors and to receptors of the immune surveillance system. We will analyze both hemphiliacs and normals for amino acid polymorphisms in the factor VIII binding domain of von Willebrand factor which may regulate the ratio of free to bound factor VIII. We propose that these polymorphisms will be found by DNA sequence analysis of individuals without clinical disease but with aberrant factor VIII/von Willebrand factor ratios. The distribution frequency of these polymorphisms among normals, non- inhibitor hemophilia patients, acquired hemophilia patients, and type I responders will be compared.